1. Field of the Invention
The invention relates generally to the field of photography, and particularly to an electronic flash control system for a camera. More specifically, the invention relates to an energy-saving flash control system which signals the readiness of the flash device for the next flash exposure.
2. Description of the Prior Art
Many commercially available electronic flash units monitor the voltage level of a flash-firing capacitor and, via a ready lamp, or by enabling a shutter release mechanism in an associated camera, inform a camera operator when there is sufficient firing voltage for a flash exposure. In some electronic flash units, the firing voltage is considered to be sufficient when the flash-firing capacitor is charged above a predetermined voltage level that corresponds to the minimum voltage required to ignite the flash tube. In other electronic flash units, the firing voltage is considered to be sufficient when the flash firing capacitor is charged above a predetermined voltage threshold that is a given percentage of the maximum operating voltage of the flash unit. Typically, the threshold voltage produces flash illumination that results in an under-exposure of about one-half to two-thirds of a stop from the flash illumination that is produced when the capacitor is fully charged.
A noteworthy electronic flash device is disclosed in commonly assigned U.S. Pat. No. 4,540,265, granted Sept. 10, 1985. The flash device enables flash exposures to be made at a rapid rate, assuming that scene conditions, such as subject range, reflectivity and ambient illuminance, do not change. When the flash energy retained by a flash-firing capacitor at the termination of the preceding flash exposure exceeds the flash energy consumed for that exposure, an energy-monitoring circuit causes a status indicator to visibly signal that the flash unit is ready for the next flash exposure.
Another noteworthy electronic flash device is disclosed in U.S. Pat. No. 4,464,039, granted Aug. 7, 1984. The flash device provides an electronic signal which corresponds to the required amount of flash light necessary to meet the flash exposure conditions, e.g. film sensitivity, aperture value, subject distance, etc. When the charge level of the flash-firing capacitor reaches a given level matching the particular level of the electronic signal, flash firing for the next flash exposure is enabled.
Although many of the known prior art control systems, such as disclosed in U.S. Pat. Nos. 4,540,265 and 4,464,039, can save energy and can shorten the waiting time to fire the electronic flash device, by relying on a lesser charge than the full charge, they are relatively complex in construction. As a result, it is difficult to employ them in a low cost camera.
A third electronic flash device which is not energy-saving, but nevertheless is relevant, is disclosed in U.S. Pat. No. 4,472,041, granted Sept. 18, 1984. Control means enables the flash device to operate in a charge mode for a fixed or constant time (always 1 sec.) immediately subsequent to termination of the exposure interval, but before a film transport motor is energized. After the elapse of the fixed time, the flash device is disabled from operating in its charge mode and the film transport motor is energized. Thus, a "sneak charge" is provided during the fixed time in order to reduce the apparent charge time subsequent to transport of the exposed film, even though the overall charge time remains unchanged. Since the photographer only perceives the flash charge time subsequent to film transport, the sense provided is one of a substantially shortened charge time for the flash device.